There are two major approaches in this study of the structural and mechanical properties of single isolated heart cells. The first method will use the properties of fluorescent antibodies that are specific to the microfilaments of heart cells to determine filament location and mechanical function within the heart cells. In this regard, the large resting tension of heart muscle remains unexplained and these microfilaments are excellent candidates for this function. In addition it is possible that radially oriented elastic elements (also microfilaments) may resist the expansion of heart cells as the heart contracts and thus limit the degree of shortening of the heart fibers. In the whole animal these two factors would affect both the filling of the heart and the degree of emptying and thus could be important factors in pathological states. In the second phase of the study, following the development of more stable transducers and control devices we will emphasize studies of the force-length and force velocity characteristics of the single intact cell that relate directly to more classical studies of heart function.